An electric motor has a rotor and an axis that are frequently provided as separate members. When they are provided as separate members, it is necessary to fix the rotor to the axis during assembly. One convenentional method is one in which the rotor is fixed to the axis using an adhesive will be described with reference to FIG. 5. FIG. 5 shows an example of a conventional method for adhering a rotor of an electric motor to be fixed to an axis. FIG. 5 shows a rotor 1, an axis 6, and adhesive agent 7. When the rotor 1 is adhered to be fixed to the axis 6, there a certain clearance is generally provided between the inner circumference of the rotor 1 and the outer circumference of the axis 6 because both the inner circumference of the rotor 1 and the outer circumference of the axis 6 have a size tolerance due to the machining process and a clearance is required not only to allow the rotor 1 to be inserted to the axis 6, but also to enable the adhesive agent 7 to have a fixed layer thickness as required by the adhesive agent employed to ensure that the desired adhesive force is provided. For this reason, a certain amount of clearance is provided between the inner circumference of the rotor 1 and the outer circumference of the axis 6, into which the adhesive agent 7 is filled when the rotor 1 is fixed to the axis 6.
Another method for fixing the rotor to the axis employs mechanical insertion or “plunge-in”. When the rotor is “plunged-in” to the axis, the size of the rotor inner circumference and the size of the axis outer circumference have a negative clearance. That is, the rotor inner circumference is on the order of a few μm to a few dozen μm smaller than the outer circumference of the axis. As a result, the rotor cannot be manually inserted into the axis by a human and a machine such as a hydraulic press is used to apply sufficient force in the axis direction to insert the rotor to the axis, thereby fixing the rotor to the axis by a frictional force caused between the rotor and the axis.
As describe above, there are basically two commonly-employed methods for fixing the rotor to the axis in the electric motor, one being to provide a clearance between the rotor and the axis which is filled with an adhesive agent for fixing with an adhesion force, and the other being to use plunged-in force to fix the rotor and the axis by mechanical friction.
When the rotor is fixed to the axis by an adhesive agent, a clearance is provided as described above between the rotor inner circumference and the axis outer circumference. This clearance causes the rotor to rotate in an unbalanced manner. Specifically, when the rotor is adhered to the axis, it is very difficult to adhere the rotor to the axis in such a manner that the clearance between the rotor and the axis is constant at any point in the rotation direction. Thus, in many cases, the rotor is adhered to the axis in a condition where the rotor is not concentric with the axis, but is off-centered. When the size of a clearance between the rotor and the axis is determined, a clearance required for the adhesive agent to provide a desired adhesive force must be considered, and the size tolerance of the rotor caused when the rotor is obtained by laminating rotor elements manufactured by a press punching also must be generally considered. When such an tolerance is considered, not only the size tolerance of the rotor inner circumference caused by aging of the tool used for the punching process, but also the lamination tolerance caused when the laminated members in the axial direction are punched to provide a rotor must be considered. If the factors as described above are all taken into consideration, the clearance between the rotor and the axis has a significant large size. Such a clearance having a significant large size causes, when the rotor is adhered to the axis in a off-centered manner, the rotor to be enormously unbalanced. When such an unbalance is increased, the amount of the unbalance is generally corrected by cutting a particular part of the rotor or by adding weight to a particular part of the rotor. However, correction of such an excessively large amount of unbalance is very difficult.
On the other hand, when the rotor is fixed to the axis by “plunge-in”, the rotor inner circumference and the axis outer circumference have a negative clearance as described above. When the rotor is inserted to the axis, the rotor is fixed to the axis while nearly equal pressures are applied to the respective points of the rotor inner circumference. Thus, the rotor is fixed to the axis while the rotor is almost concentric with the axis, and the center of the rotor commonly is no more than slightly displaced from the center of the axis. As a result, the amount of unbalance caused by the “plunge-in” method is very small compared to the adhesion method. However, when a reluctance motor has a rotor in which a plurality of slits for providing a magnetic pole are provided, another problem results, as will be described with reference to FIG. 6, which is an enlarged view of a portion of FIG. 5. The “plunge-in” method inserts the rotor to the axis while deforming the rotor inner circumference, thus applying an enormously large force to the rotor inner circumference. If the reluctance motor has a rotor in which slits are provided, the existence of the slits weakens the entire structure with respect to centrifugal force. Thus, when such an enormously large force is applied to the rotor inner circumference, the force has a significant influence on the rotor inner circumference.
In particular, a force is applied to the component denoted as “P6” in FIG. 6 in the direction “F6” because the distance between the inner circumference of the rotor 1 and the slit 3 is very small. However, the part P6 has a thin thickness to which the force is applied and, thus, the force applied to this part also has an influence on other parts of the rotor 1. For example, both connection parts shown by “2D” are applied with the force in the direction shown by “F7”. Furthermore, when the rotor 1 is rotated to receive a centrifugal force, the stress applied to the connection part 2D is increased enormously. This stress exceeds a limitation of allowable stress of the material, thus breaking the connection part 2D.
In order to prevent this, either the part P6 or the connection part 2D must be made thicker for reinforcement. However, increasing the thickness of the part P6 requires the rotor 1 to have an increased outer diameter or other magnetic paths or the slit 3 to have a reduced thickness. However, in most cases, the outer diameter of the rotor 1 has an optimal relationship in size with the rotor provided at the exterior of the rotor 1. Thus, increasing the outer diameter of the rotor 1 leads to a decline in the generated torque and output. Reducing the thickness of other magnetic paths or the slit 3 in the rotor 1 also leads to a decline in generated torque and output. On the other hand, increasing the thickness of the connection part leads directly to an increase in the leakage of magnetic flux and thus also to a decline of generated torque and output. As described above, the “plunge-in” method to fix the rotor to the axis requires a particular part to have an increased thickness for reinforcement, thus causing a decline in the characteristics of the electric motor.